Vapor Intrusion from Subsurface to Indoor Air:

Transcription

1 Vapor Intrusion from Subsurface to Indoor Air: Biodegradable d Petroleum Vapors versus Recalcitrant t Chemicals Vapor Intrusion 2010: Air & Waste Management Association (A&WMA) Conference Chicago, IL. Dates: September 29-30, 2010 Use this area for cover image (Maximum height 6.5cm & width 8cm) George DeVaull george.devaull@shell.com 1

2 To Cover Conceptual Models Compartments: Building; Foundation; Soil Layers; Vapor Source What are conditions for potential worst-case case indoor air impacts? For non-degrading chemicals For aerobically degrading g petroleum eu hydrocarbons Worst-case occurs at extremes of parameters not averages Helps reconcile models and observed behavior trends Footer: Title may be placed here or disclaimer if required. May sit up to two lines in depth. 2 2

3 Conceptual Model: Non-degradable chemicals What are potential worst case conditions? Building: High vapor resistance Closed up. Low air exchange rate. Holds vapors indoors. Foundation Low vapor resistance High vapor migration through floor. Dirt floors, cobble foundations, open cracks, etc. Worst-case is NOT poured concrete foundation Soils and Source Air-filled soils. Low vapor resistance smaller separation, higher h concentrations ti 3

4 Foundation Resistance Advective flow: Pressure gradients across foundation drives airflow. Diffusive flow: Chemical flow driven by concentration gradients total t flux = diffusion i + advection 10 Pa = 1.02 mmh 2 O ~10-4 atm Plot: Patterson & Davis, Environ. Sci. Technol., 2009, 43, Flow both ways Cyclic fluctuating flow through a foundation causes advective mass transfer, even if time-averaged airflow is zero. The foundation breathes. Below foundation air moves into indoors, and Indoor air moves below the foundation One-way pressure-driven flow from soil through h foundation to indoors not confirmed by measurement 4

5 Foundation Resistance Could be either (or both) Advection or Diffusion Advective Flow Diffusive Flow Q = Equivalent airflow Foundation Area, A f D eff = Effective Diffusion coefficient Foundation Thickness, L f Either inferred from concentration measurement Either way, chemicals move across the building foundation: 5 5

6 Equivalent Airflow Through Foundation Empirical Field Data Summary Total range: 0.23 to 60 L/min bare and dirt floors, cobble foundations, highly permeable foundations, depressurized buildings 0.23 to 15 1 to 10 (avg.) 0.9 to to 60 concrete slab on grade, concrete basement, enclosed crawl space with vapor barriers, concrete pavement diffusion through air dry concrete (10 to 20 cm thickness) Equivalent Air Flow Rate (L air/minute) for 100 m 2 foundation Broad foundation range reconciles indoor/sub-slab data variability. Potential Worst case is at extremes, not at the average. Average Value Average Indoor/Subslab Attenuation Factor Worst-case Value Worst-Case Indoor/Subslab Attenuation Factor 6

7 Biodegradable Hydrocarbons: Evidence for Degradation z 0 = profile knee, below which: (low) zero oxygen, water table, residual oil, or high organic soil Exponential concentration decrease above z 0: C (z) / C 0 = exp (- z / L R ) L R = log slope: 0.6 ft ( 0.16 to 1.6 ft ) Median [range] L R = Reaction Length Distance to exp(-1) ~ Key: Vadose Biodegradation Rates are Large (with O 2 Present) Greater aerobic depth, more attenuation 7 7

8 Conceptual Model: Aerobically Biodegradable chemicals What are worst case conditions? Compared to no-biodegradation Aerobic Biodegradation reduces indoor air concentration levels. Less Biodegradation Less oxygen in shallow subsurface Shorter aerobic depths Occurs for: Greater oxygen demand. Greater petroleum impacts Higher foundation resistance (if O 2 limited). i Key Ideas: Worst Case conditions i for Building, Soils and Source, same as non-degrading di case Worst Case Foundation Type is the Opposite Extreme 8

9 Trends Effect of Equivalent Foundation Airflow Base Case Exclusion Distance : 5 ft depth, water-dissolved source 1 mg/l benzene, 10 mg/l BTEX R. Davis (2010) or Air Benz zene Concen ntration (ug g/m 3 ) Without Biodegradation Higher foundation airflow, Higher indoor air concentration With Aerobic Biodegradation Higher foundation airflow, Lower indoor air concentration Indo (if oxygen limited) Equivalent Foundation Airflow (L/min) Biodegradation d i neglected Biodegradation included Below nominal indoor air background Soil Layer Entirely Aerobic Model Estimates (BioVapor, Residential default parameters 9

10 Summary Potential worst case indoor air conditions? Building: Low Air Exchange. Smaller Mixing Height Foundation: Effective Airflow Soils Non-degrading d Chemicals: 37 to 60 L/min (High) Aerobically Degrading: 0.23 to 1 L/min (How) For 100 m 2 foundation Air-filled soils. Sand Soils. Smaller Separation Distance. Source Higher Source Vapor Concentrations. 10

11 Summary: Key Ideas Foundation: Simpler description Effective Airflow per Unit Area: 0.23 to 60 L/min for 100 m 2 foundation Advection or Diffusion not differentiated Bounded Range Compartment Level Evaluation: Building, Foundation, Soils, Source Sensitivity Checks: Permutation from Screening Criteria (example) Which compartment has the greatest variability? Choose assessment or mitigation choices for most sensitive compartments t 11

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